This invention relates to a mobile communications receiving apparatus and method. More particularly, the invention relates to a mobile communications receiving apparatus and method for combining and receiving signals that have been received on a plurality of receiving branches.
A diversity combining reception method described in, e.g., “Demodulation of Digital Wireless Communications” by Yoichi Saito, published by the Electronic Information Communications Society (a corporate juridical person), First Edition, Third Printing, Sep. 1, 1997, pp. 189–191, is known as a technique involving a diversity combining receiver circuit in a mobile communications system.
In the reference mentioned above, the maximum ratio combining method is indicated as one method of diversity combining reception. According to the maximum ratio combining method, signals that have been received on a plurality of receiving paths (branches) are combined upon being weighted by the respective envelope levels (amplitude levels). FIG. 11 is a diagram useful in describing the maximum ratio combining method. Here a first receiving path (first branch) is formed by an antenna 1a and a receiver 2a, an envelope level detector 3a detects the envelope of the receive signal in the first branch and a gain varying unit 4a multiplies the receive signal by a gain g1, which conforms to the envelope level, and outputs the resultant signal. A second receiving path (second branch) is formed by a directional antenna 1b and a receiver 2b, an envelope level detector 3b detects the envelope of the receive signal in the second branch and a gain varying unit 4b multiplies the receive signal by a gain g2, which conforms to the envelope level, and outputs the resultant signal. A phase detector 5 detects the phase difference between the receive signals in the first and second branches, a phase shifter 6 shifts the phase of the receive signal in the second branch by an amount equivalent to the phase difference to thereby match the phases of the receive signals of the two branches, a combiner 7 combines the receive signals of the two branches weighted by the envelope levels, and a demodulator 8 demodulates the transmit data based upon the combined signal.
The maximum ratio combining method which, under certain conditions, maximizes SNR (Signal-to-noise power ratio) after the signals are combined, is in widespread use.
However, the maximum ratio combining method is the optimum combining method on the assumption that noise components contained in the receive signals of the respective branches have no mutual correlation. Accordingly, the maximum ratio combining method is not necessarily the best combining method in circumstances where it cannot be assumed that the noise components contained in the receive signals (referred to also as “branch signals” below) of each branch are not mutually correlated. It is possible that using this method to combine signals may worsen characteristics rather than improve them.